Uniformed fees

So an account of mine got handed over to these people without even ever receiving an invoice from them. I settled the account and then got a call again regarding an second invoice, so I made a payment arrangement to settle the amount in 2 payments. Never was I advised of the terrible fees that will be added. After the second payment was made and the account settled to my knowledge I get a phone call saying that I owe more money, this amount now being for fees. In a matter of 2-3 weeks this fees amount went from R118.80 to R316. How can this even be possible? The call agents are harassing me at work and will be the cause of me losing my job with the constant phone calls. I can not be held liable for fees which I was never informed about. and how can the amount for fees grow so fast?